1. Field of the Invention
The invention relates to an apparatus for measuring the capacitance value of a low value three terminal capacitor. More specifically, the invention relates to such an apparatus which contains a means for maintaining a point in the circuitry of the apparatus at virtual ground.
The means for maintaining the point at virtual ground is preferably an active guard circuit. The invention also relates to an instrumentation amplifier which may be used as the guard circuit. The instrumentation amplifier may be used for other purposes as well.
2. Description of Prior Art
The apparatus of the present invention comprises a resonant system using an active guard circuit and using a substitution method to compare the capacitance value of unknown three-terminal capacitors (3TC) with the capacitance value of a variable standard 3TC that could be operated in the frequency range 50 kHz-10 MHz for making dielectric measurements.
Dielectric measurements in this frequency range are commonly made with home built resonance circuits (M. D. Benadda, J. C. Carru, and C. Druon, J. Phys. E 15, 132 (1982); I. Ogawa and A. Kakimoto, Rev. Sci. Instrum. 49, 936 (1978); and B. Ichijo and T. Arai, Rev. Sci. Instrum. 32, 122 (1961)). This is because resonant methods generally lead to measuring systems which are simple, accurate, easy to calibrate, and which use instruments already found in most laboratories such as frequency meters, oscilloscopes, and signal generators. In contrast, bridge method systems which would permit precise measurements of low value capacitors in the radio frequency range are rather elaborate (L. D. White and R. C. Strum, Bell Syst. Tech. J. 60, 405 (1981)), since they either involve many conductance and capacitance standards with low parasitics or a few standards and precise phase-sensitive detection which is difficult to achieve in the MHz range. Because they are designed for a 50 ohm reference, commercially available impedance analyzers are not very practical for small capacitance measurements in the MHz range. Transmission line sampling, with digital data processing has been used and can be improved for measurements from 1 to 200 MHz or even 10 GHz, with reasonable precision on both capacitance and dissipation factor, as can time domain reflectometry (R. Chahine and T. K. Bose, Rev. Sci. Instrum. 54, 1243 (1983); R. H. Cole, IEEE Trans. Instrum. Meas. IM-32, 42 (1983) and B. Gestblom, J. Phys. E 15, 87 (1982)). However, transmission line methods permit measurement of two terminal devices only, and they can never match the precision of a three terminal measurement (J. F. Hersh, General Radio Experimenter, Vol. 3, 1 (1959)).
Turning to three-terminal measurements (3TM), as there is no point in the circuit being naturally at virtual ground, resonance methods do not lend themselves to 3TM. Since 3TM's are superior to ordinary two terminal measurements for dielectric measurements up to 10 MHz (R. Courteau and T. K. Bose, Rev. Sci. Instrum. 58, 1096 (1987)), the development of an active guard circuit to maintain a point in the circuit at virtual ground would facilitate the use of the three-terminal configuration over its upper useful frequency range. The benefits of such a system for dielectric measurements are that: (1) it enables the use of 3TC's and three-terminal cells, simplifying calibration procedures and permitting higher accuracy; and (2) it permits the use of flexible coaxial cables of arbitrary length for making the connections (although the length is limited at high frequencies), since the parallel capacitance of the cables becomes part of the terminal impedances, which do not influence the result for an ideal 3TM. While bridges may be designed so that no additional circuits are required to do 3TM's (J. G. Berberian and R. H. Cole, Rev. Sci. Instrum. 40, 811 (1969)), other methods generally need a guard circuit to ensure that terminal impedances are not part of the measurement.
Precise dielectric measurements within the frequency range 50 kHz-10 MHz are important for the study of dielectric relaxation of polymers and biological systems, ionic relaxation in microemulsions, and Maxwell-Wagner absorption in emulsions.